High current, high mechanical strength connectors for insulated metal substrate circuit boards

ABSTRACT

The invention is a method of and devices for making electrical connections to an Insulated Metal Substrate (IMS) printed circuit board. The approach is novel. The invention enables IMS type materials to be cost-effectively used with much higher currents. The invention also enables the substrate layer to be used as an active circuit layer.

BACKGROUND OF INVENTION

[0001] This invention pertains to power electronics design and manufacture. Specifically, the invention pertains to high current electrical connector designs that provide connections to Insulated Metal Substrate (IMS) circuit boards. IMS circuit board materials are comprised of a metal substrate, usually aluminum or copper with a typical thickness from 0.040″ to 0.125″. A thin insulating material is bonded to the substrate and a layer of copper foil is bonded to the insulating material. The IMS material is processed into printed circuit boards (PCBs) in much the same way as a typical fiberglass PCB where a photo mask is applied to the copper foil and the unwanted copper is chemically etched away, leaving the desired traces and pads. For high power applications, IMS printed circuit boards have only one usable layer and are only suitable for surface mount components. Fiberglass boards can have many layers and a mix of through-hole and surface mount components. The value of the IMS material is in the very low thermal resistance from copper component mounting pad to the metal substrate. In high power applications, the substrate is in turn mounted to a heatsink. This allows surface mount semiconductor components such as transistors, rectifiers and SCRs to operate with low thermal resistance from device junction to ambient air. Low thermal resistance enables higher power to be processed with less silicon die area, which translates to lower costs. The manufacturing benefits and reliability enhancements of pick-and-place assembly is known. There are two problems with the IMS materials for high power applications. First, the mechanical strength of the bond between the copper and insulating material and insulating material and substrate is limited. This weakness precludes the use of large soldered, surface mounted terminals that require high sheer and pull strength to reliably hold large cables. The prior art is to use multiple low-current surface mount connectors, pins or headers to make the transition to a fiberglass PCB where a single, high current, high mechanical strength termination can be made. Second, the metal substrate layer is typically used only for mechanical support and heat transfer. It is desirable in most high power applications to have a low impedance DC bus which requires a two layer circuit board or other laminated bus assembly. The IMS material is limited by having only one easily accessible layer. In lower power applications an IMS board can be configured with a second copper layer but the heat transfer characteristics are compromised and the added cost may be prohibitive. Additionally, the problem of connecting large through-hole electrolytic capacitors to the to the circuit layers to achieve a low AC impedance bus is not solved.

SUMMARY OF INVENTION

[0002] The invention solves the two problems stated in the background discussion. First, a device will be disclosed that allows a high current connection to be made from an IMS printed circuit board to a wire or to a second printed circuit board. The connection will have high sheer and pull strength and is independent of the IMS insulating material bond strength. Second, a device and method will be disclosed for making a high current, low impedance, electrical connection from both the top copper foil and the IMS substrate to a second printed circuit board. This allows the IMS substrate to be used as an active circuit plane in conjunction with all or part of the top IMS copper foil to create a low AC impedance bus structure. This also allows a low AC impedance coaxial connection with a fiberglass board that can carry the larger through-hole components, such as electrolytic capacitors, relays and terminal blocks that the IMS board cannot.

DETAILED DESCRIPTION

[0003] The invention consists of three electromechanical components; a high current PCB terminal, a high current coaxial connector link, a high conductivity locknut and the application of said components. The high current PCB terminal, illustrated in FIG. 1 is made up of three parts, a metal cap 1 with a threaded machine screw stud on the top of the cap, and electrically insulating material 2 and a threaded machine screw insert 3. The preferred embodiment would use a potting compound for the insulating material 2. FIG. 4 illustrates a typical application for the high current PCB terminal where a low resistance connection is made between a fiberglass PCB 30 and an IMS PCB 40. The top copper foil layer 41 of the IMS PCB 40 is etched to remove the copper foil within a prescribed radius to provide voltage clearance between a machine screw stud 50, which is press-fit into the metal substrate of the IMS PCB 40. The terminal 1 is screwed onto stud 50 making electrical contact with the top foil 47 of the IMS PCB. In the preferred embodiment, terminal 1 is soldered to the top foil of the IMS PCB around the bottom outside circumference of terminal 1 or under the mating surface ring area using re-flow solder paste. This is now a high current, high strength terminal that is isolated from the IMS substrate material. A wire with a ring terminal can now be fastened here. If a hole is drilled thru the fiberglass substrate 37 and the bottom foil 32 of the fiberglass PCB 30 and if fiberglass PCB 30 is fastened to terminal 1 with standard flat washer 23, lock washer 22 and hex nut 21, a solid electrical compression contact is made between terminal 1 and the bottom foil 32 of fiberglass PCB 30. This assembly provides a reliable, high current connection between the top foil of the IMS PCB and the bottom foil of the fiberglass PCB and also provides a means of mounting the fiberglass PCB. The high current coaxial connector link illustrated in FIG. 2 is made up of two parts; an outside metal ring 1 and a bushing 2 made of an electrically insulating material. The bushing 2 has a center through-hole. The high conductivity locknut illustrated in FIG. 3 is fabricated in one piece with an outside width and thickness much larger than standard hex nuts in proportion to the thread size to extend the bearing surface without using a fender washer. The threads are slightly deformed to provide lower electrical contact resistance and to lock the nut in position. The base metal and plating materials are selected for high electrical conductivity and corrosion resistance. A typical application for the high current coaxial connector link and the high conductivity locknut is illustrated in FIG. 5 where the substrate of the IMS board is used as an active conductor and where two, high current conductors are brought from an IMS PCB to a fiberglass PCB with very low AC impedance between the two conductors. Coaxial link 2 is placed over machine stud 50, fiberglass PCB 30 is placed over machine stud 50 and the assembly is fastened together with high conductivity nut 3. The resulting electrical circuit is a low resistance connection between the top foil 41 of the IMS PCB 40 through the coaxial link 2 to the bottom foil 32 of fiberglass PCB 30 and a second circuit with a swaged connection between the IMS PCB substrate 43 the press-fit stud 50, through nut 3, to the top foil 34 of the fiberglass PCB. The AC impedance between these two current paths is very low due to the geometry of the conductor link 2. This method allows the use of the IMS substrate layer as an active power plane to form a low AC impedance bus structure on the IMS PCB and allows this low AC impedance characteristic to be maintained between the IMS PCB and the energy storage components on the adjacent fiberglass PCB without stressing the mechanical bonds of the IMS insulating layer 42. 

1. A device that makes an electrical connection to the top foil of an insulated metal substrate (IMS) printed circuit board (PCB) using a mechanical fastener and the structural integrity of the IMS substrate to simultaneously provide a high strength mechanical connection and an electrical connection that is electrically isolated from said fastener and said IMS substrate.
 2. A method of making two simultaneous electrical connections from one Printed Circuit Board (PCB), of any material type, to another PCB, of any material type, using four basic components; a conductive ring; an electrically insulating bushing having a center through hole; a machine screw and a nut. The said bushing is placed inside the said conductive ring and both are sandwiched between two PCBs, each having at least one through hole. The assembly is clamped together with the machine screw and nut. The conductive ring carries the current of one of the electrical circuits and the machine screw carries the current of the other circuit.
 3. A method according to claim 2 where the said electrically insulating bushing has more than one through hole.
 4. A method according to claim 2 where the machine screw is replaced by a self-clenching threaded stud.
 5. A method according to claim 2 where the machine screw and nut are replaced by other common fasting devices which provide the same function of providing adequate pressure on all of the intended electrically conducing surfaces to provide reasonably low contact resistance for the intended application.
 6. A device for enabling two simultaneous electrical connections from one Printed Circuit Board (PCB) to another PCB that is comprised of a conductive ring and an electrically insulating center bushing having at least one through hole.
 7. A device for enabling two simultaneous electrical connections from one Printed Circuit Board (PCB) to another PCB that is comprised of a conductive nut with a relatively large bearing surface area, special threads and is intended for use with the device according to claim
 6. 8. A method according to claim 2, using IMS type materials, allowing the IMS substrate layer to be used as an active circuit layer. 